1. Field of the Invention
The present invention relates to method for manufacturing a porous body of a stainless steel powder by sintering the stainless steel powder and, more particularly to a method which permits sintering of the stainless steel powder without applying any pressure to the stainless steel powder at a temperature below its melting point to obtain a porous body which is excellent in mechanical strength, corrosion resistance and heat resistance.
2. Description of the Prior Art
Heretofore, a porous body obtained by sintering an iron powder, copper powder or the like has been used as an oil, water or similar liquid filter, but in recent years the porous body is receiving particular attention as a sound-absorbing material since the porous structure of the porous body has excellent sound-absorbing properties. Although the porous body is such a useful industrial material, porous bodies now placed on the market are sintered bodies of iron, copper and like powders and these sintered bodies are poor in corrosion resistance and heavy, and hence limited in use. To avoid the abovesaid defects, the present inventors have previously proposed a method for sintering a porous body of a lightweight aluminum powder. The porous body obtained by this method has a pore ratio of, for example, 40% or more and exhibits very excellent sound-absorbing properties but is poor in heat resistance and in mechanical strength.
In contrast thereto, a porous body of a stainless steel powder is excellent mechanical properties and rich in corrosion resistance and in heat resistance, and hence is preferred as a sound-absorbing material. Since the stainless steel powder has a large hardness of, for example, H.sub.RC 40 to 50 or so and has a high sintering temperature, however, the sintering method itself poses a problem. According to a prior art method for sintering the stainless steel powder, the powder is heat treated prior to sintering, to reduce its hardness to less than H.sub.RC 40, and the powder is formed by rolling into a compact body, which compact body is then sintered at such a high temperature as 1300.degree. to 1400.degree. C. This conventional method involves preheat treatment of the stainless steel powder and, moreover, the high sintering temperature requires expensive sintering facilities and raises the sintering cost and, moreover, a porous body of the stainless steel powder can not be obtained by the conventional method; accordingly, there is a strong demand for improvement of the sintering method.
The present inventors had make a study of a sintering method which would permit sintering of the stainless steel powder at a relatively low temperature regardless of the shapes of powder particles to obtain a porous body of excellent mechanical strength and corrosion resistance. As a result of their study, they have proposed a method in which one or more of Cu-Mn or Ni-Mn alloy powders are mixed in the stainless steel powder and the powder mixture is loosely packed into a required configuration and sintered in a non-oxidizing atmosphere at the melting point of the alloy powder or at a higher temperature. With this method, the sintering temperature is relatively low and the pore ratio of the sintered body can freely be adjusted.